The present invention relates to a process of operating a sliding closure unit, particularly of the three-plate type, to control the discharge of molten metal from a metallurgical vessel through a discharge passage of the sliding closure unit into a mold to regulate the level of the molten metal within the mold by moving a sliding plate of the sliding closure unit to selected throttling positions thereof relatively restricting the size of the discharge passage. The present invention particularly relates to such a process wherein the movement of the sliding plate to regulate the level is controlled automatically as a function of the level in a level control operation by a processor, and wherein periodically the level control operation is automatically interrupted and the sliding plate is moved in a throttling position change operation from one throttling position through a completely open position of the discharge passage to another throttling position, such two throttling positions employing throttling surfaces of the sliding closure unit, and specifically of the sliding plate, on opposite sides of the discharge passage.
In practical molten metal casting operations, such as continuous casting operations, such a change of throttling positions, i.e. between opposite edges or surfaces of the sliding plate, results in the discharged molten metal operating on different surfaces of the sliding closure unit. As a result, erosion of the various surfaces of the sliding closure unit tends to be more uniform over the various involved surfaces, and this results in increased service life of the elements of the sliding closure unit. Furthermore, it is important that the discharge channel through the sliding closure unit be kept free of deposits that tend to form within the discharge channel, German application P 37 42 215.4 (corresponding to U.S. Pat. application Ser. No. 281,053 filed Dec. 8, 1988 and now U.S. Pat. No. 4,890,665 achieves a deposit-free discharge channel and also improved uniformity of erosion by periodically moving a sliding plate from one throttling position through a completely open position of the discharge passage to another throttling position, preferably employing opposite surfaces of the sliding closure unit. This throttling position change operation is directed by a processor while such level control operation is switched off. This achieves a "rinsing" effect tending to wash away any deposited solids and also tends to achieve uniform wear of the surfaces of the sliding closure unit that achieve throttling.
This throttling position change operation from one throttling position to an opposite throttling position however causes certain problems. This particularly is true with regard to sliding closure units used to control the discharge of molten metal into a continuous casting mold. Thus, when the sliding plate is moved from one throttling position through a completely open position of the discharge passage to another throttling position, there is an instantaneous increase in the flow of molten metal through the discharge passage. Additionally, the throttling position to which the sliding plate is moved is not always appropriate for the particular level existing at that moment in the mold. As a result of both of these factors, substantial fluctuations in the molten metal level often occur, and this can significantly influence the casting operation. Particularly, since the level control operation is interrupted during the throttling position change operation, the return to an accurate level control operation is rendered difficult.